1. Field of the Invention
The present invention relates to a cooling architecture for flanges of a steam turbine casing in order to prevent the leakage of steam caused by a drop of the fastening force of bolts for fastening the flanges.
2. Prior Art
FIG. 4 is a sectional view illustrating a portion of the casing of a conventional steam turbine, wherein reference numeral 10 denotes an upper casing, and 11 denotes a lower casing. A steam turbine is hermetically sealed by these two casings to prevent the leakage of the steam to the outside. Reference numerals 12 and 13 denote flanges of the upper and lower casings 10 and 11. The two flanges 12 and 13 are joined together and are fastened by bolts 14 which are arranged every predetermined interval along the axis of the turbine to couple the upper and lower casings 10 and 11 together. Reference numeral 15 denotes a heat insulator which covers the surfaces of the flanges 12 and 13, upper parts of the bolts 14 and the surfaces of the upper and lower casings 10 and 11 as shown.
In the casing of the steam turbine constituted as above, because the steam of a high temperature flows through the inner steam turbine, the casing is heated by high temperature steam, and the flanges 12 and 13 are also heated and are thermally deformed. Upon receiving the thermal deformation, the bolts 14 undergo the thermal extension and gradually decrease the fastening force after repetitions of the above-mentioned cycle. As the fastening force drops, the steam leaks through the junction surface between the flanges 12 and 13. Because the temperature of the steam is high, the leakage of the stream is dangerous. Besides, large amount of the leakage of steam affects the performance of the steam turbine.
In order to prevent the leakage of steam, there has heretofore been employed a cooling architecture shown in FIG. 5, wherein reference numerals 10 to 15 denote the same elements as those of FIG. 4. In this case, however, holes 25 are formed in the vertical direction to penetrate through the flanges 12 and 13 and the heat insulator 15 near the bolts 14 in order to prevent the bolts 14 and the flanges 12 and 13 near the bolts from being thermally deformed. Because the peripheries of the holes 25 are heated to a high temperature by the steam, natural convection flow of the ambient air 30 is generated to spontaneously cool the portions of the flanges around the bolts 14.
In the conventional casing of the steam turbine as described above, the casing, too, is heated to a high temperature due to the high-temperature steam, the bolts for coupling the flanges are thermally deformed to gradually decrease the fastening force, and the steam may leak through the junction surfaces of the flanges. As shown in FIG. 5, therefore, holes 25 are formed in the flanges 12, 13 and in the heat insulator 15 around the bolts 14, in order to cool the bolts 14 and the flanges 12 and 13 around the bolts based on the natural convection of the air.
According to the above-described conventional method, that is, perforation of the flanges 12 and 13, however, holes must be pierced through not only the flanges 12 and 13, but also the heat insulator 15, and laborious work for piercing the holes is required. Besides, the holes are clogged with the dust of the heat insulator and the air is not often naturally convected to a sufficient degree, and some countermeasure must be taken.
The present invention, therefore, provides a cooling architecture which reliably cools the flanges of the steam turbine casing based on the natural convection of the air, and by forming holes through the heat insulator, but not through the flange to create the natural convection of the air with a simple process.